This invention relates to a sample nebulization apparatus. More specifically, this invention relates to a new and improved ultrasonic nebulization apparatus for generating aerosols suitable for use in atomic spectroscopy, simulation of industrial and environmental atmospheres, for inhalation therapy or exposure, and for determination of filter efficiency.
Atomic spectroscopy, which includes atomic emission, absorption and fluorescence spectroscopy, requires that the sample for analysis, generally present in solution or suspension, be atomized or transformed into an aerosol so that the sample can be transported to an excitation source, such as a flame or plasma torch, where the sample is converted to atomic or ionic species for atomic spectroscopic analysis.
Pneumatic nebulization has generally been used for aerosol formation for analytical purposes. However, the use of pneumatic nebulization has a number of drawbacks which affect aerosol formation and which may have an adverse effect upon the analytical results obtained with the method. For example, pneumatic nebulization is often very inefficient, only about 1% of the sample volume actually reaching the atomic spectroscopy excitation source. Effective nebulization and droplet formation require a high rate of flow of nebulizing gas. However, since the nebulizing gas is also the aerosol carrier gas, the flow rate is too high for some excitation sources such as plasmas. A high flow rate will also impact some of the aerosol on the walls of the chamber, resulting in loss of sample. Pneumatic nebulizers that operate at low flow rates have been developed, but their efficiency of nebulization is low and they have a tendency to clog. Solid materials which may be present in sample solutions or suspensions often cause clogging of pneumatic nebulizers. In this event, the nebulizer-spray chamber apparatus usually has to be disassembled and cleaned in order to regain optimum nebulizer performance.
Ultrasonic nebulizers possess a number of attractive characteristics. First, the rate of aerosol production at the transducer does not depend on the carrier-gas flow as it does in pneumatic nebulizers. Thus the aerosol production rate and the carrier-gas flow rate may be varied independently. Second, ultrasonic nebulizers can produce aerosols of greater number density and of more uniform particle size than pneumatic nebulizers. Third, the mean size of the particle produced by an ultrasonic nebulizer is frequency dependent; smaller particles can be produced by increasing the ultrasonic frequency employed. The advantage gained is that smaller particles are more efficiently transported and are more rapidly desolvated and atomized in the excitation cell.
Two types of ultrasonic nebulizers have generally been used with atomic spectroscopy excitation sources. In continuous feed ultrasonic nebulizers, the analyte solution is continuously pumped onto either the transducer surface or transfer plate, directly connected to a transducer, where the analyte is nebulized by direct impingement. These nebulizers are preferred for routine analysis because rapid sample interchange can be achieved and because the sample cleanout time, necessary to avoid memory effects or the reintroduction of a previous analyte, is acceptably short. Solutions of high salt content or of high acidity or alkalinity have been found, however, to attack the transducer, even when various protective coatings were employed. The use of O-ring sealed transfer plates to nebulize the analyte and protect the transducer has also been tried, but this approach either suffers from seal failure, where the analyte ultimately reaches and attacks the transducer, or from incompatibilities between the analyte plate material and analyte ions in the sample solution. In batch type ultrasonic nebulizers, the ultrasonic energy is coupled to a known initial volume of analyte solution, either directly or through an inert liquid or solid interface, thus circumventing the corrosion problem. These nebulizers suffer from other experimental shortcomings, however. Foremost among these are inconvenient sample change capability and memory effects.